1. Area of the Invention
The present invention relates to a circuit configuration having a feedback operational amplifier for amplifying an input signal input into the circuit configuration and outputting the amplified input signal as an output signal.
Circuit configurations of this type are known in manifold forms. Solely for exemplary purposes, reference is made to the textbooks “U. Tietze, Ch. Schenk, Halbleiter-Schaltungstechnik [Semiconductor Circuit Technology], 5th Edition, Springer-Verlag, 1980”, in particular pages 93 through 108, and “Paul Horwitz, Winfield Hill, The Art of Electronics, Second Edition, Cambridge University Press, 1989”, in particular pages 175 through 195.
The term “operational amplifier” as defined in the present invention is to be understood very broadly as a configuration capable of amplifying an electrical variable such as a voltage. In particular, it refers to amplifiers, for example, in which a signal applied to the amplifier input is provided having relatively high voltage amplification at the amplifier output. This open amplification (“open loop again”) may be in the magnitude of approximately 104 through 105, for example.
An essential characteristic of the circuit configuration according to the species is that the circuit amplification, i.e., the ratio between output signal and input signal, is practically completely independent of this open amplification (except for transient occurrences) and is solely predefined by an additional (external) configuration of the operational amplifier.
The term “feedback network” in the circuit configuration according to the species refers to any such configuration of the operational amplifier which exerts an influence on the resulting circuit amplification. In the simplest case, the feedback network comprises a configuration of one or more (ohmic) resistors. Alternatively or additionally, other components such as capacitors and/or inductors may be provided to produce the feedback network. Very generally, these components provided to produce the feedback network are thus identified in the following as impedances.
2. Description of the Prior Art
There are applications in which a circuit configuration is required in which the circuit amplification is changeable or even continuously changeable. In order to achieve this, essentially two approaches are followed in the prior art.
One possibility is to connect a damping element, whose damping is changeable, upstream of an amplifier having a fixed predefined amplification.
Another possibility is to implement amplification-determining resistors of the feedback network on an operational amplifier by MOS transistors, which are operated in the triode range. The effective resistance of these transistors may be changed continuously by a corresponding change of activation signals (gate source voltages).
Both achievements of the object are not satisfactory in all applications. In the first achievement of the object, nonlinearities typically occur in the damping element, for example, which have an effect on the circuit amplification.
In the second achievement of the object, above all, a disadvantage is the restricted linearity of the resistors implemented by MOS transistors in the triode range and, accompanying this, a reduced linearity of the circuit configuration.
In addition, a further disadvantage of the known circuit configurations is that their output signal is often impaired by interference signals, particularly noise unavoidably occurring at the input side of the operational amplifier, for example. In a resistive feedback amplifier configuration, the extent of the “white noise” (flat curve of the frequency-dependent noise output), which is at least to be observed as dominant, such as the noise output or the noise voltage occurring at a resistor element, is a function of the relevant resistance values(s). A greater resistance value results in a greater effective noise voltage. The use of resistor elements having resistance values in the magnitude of approximately 100 Ω or less is to be derived from this law for a circuit configuration implemented in CMOS technology for many applications (if noise up to a few nV/(Hz)0.5 is to be permitted). Although resistance values this low may be implemented in CMOS technology, it is extremely difficult to change resistance values of this type in operation at great precision or with high linearity in relation to an activation signal in order to change the circuit amplification precisely during operation of the circuit configuration.